RESPIRATORY-DRIVEN NA+ ELECTRICAL POTENTIAL IN THE BACTERIUM VITREOSCILLA

Vitreoscilla is a Gram-negative bacterium with unique respiratory physiology in which Na+ was implicated as a coupling cation for the generation of a transmembrane electrical gradient (Δψ). Thus, cells respiring in the presence of 110 mM Na+ generated a Δψ of −142 mV compared to only −42 and −56 mV for Li+ and choline, respectively, and even the −42 and −56 mV were insensitive to the protonophore 3,5-di-tert-butyl-4-hydroxybenzaldehyde (DTHB). The kinetics of Δψ formation and collapse correlated well with the kinetics of Na+ fluxes but not with those of H+ fluxes. Cyanide inhibited respiration, Na+ extrusion, and Δψ formation 81% or more, indicating that Δψ formation and Na+ extrusion were coupled to respiration. Experiments were performed to distinguish among three possible transport systems for this coupling: (1) a Na+-transporting ATPase; (2) an electrogenic Na+/H+ antiport system; (3) a primary Na+ pump directly driven by the free energy of electron transport. DCCD and arsenate decreased cellular ATP up to 86% but had no effect on Δψ, evidence against a Na+-transporting ATPase. Low concentrations of DTHB had no effect on Δψ; high concentrations transiently collapsed Δψ, but led to a stimulation of Na+ extrusion, the opposite of that expected for a Na+/H+ antiport system. Potassium ion, which collapses Δψ, also stimulated Na+ extrusion. The experimental evidence is against Na+ extrusion by mechanisms 1 and 2 and supports the existence of a respiratory-driven primary Na+ pump for generating Δψ in Vitreoscilla. © 1990, American Chemical Society. All rights reserved.